@chris_b A bunch of stuff. I am a backend developer. I have to take input from internet over http(s). I have to talk to databases, restful services, soap services, write logs. I have to deal with transactions. Some of those things are OS level things. Some of those are standard protocols. Developers of my favorite language can of course implement all of those. And I can implement some of those myself. Of course, those implementations are fairly likely to be much less efficient, bug free and fully featured as the mature ones present in JVM or .NET.
For an android developer the list of things they need will be slightly different (my guess is that it will involve drawing stuff on the screen), but the basic premise is the same. Having access to the enormous Java landscape gives you a lot of power and, from my point of view, should be seen as a language feature much more important than syntactic sugar like Data classes or Smart casts.